1. Field of the Invention
This invention relates to the liquid-phase oxidation of an alkyl aromatic with an oxygen-containing gas at an elevated temperature and pressure and in the presence of an oxidation catalyst. More particularly, it relates to an improvement in the process for the oxidation of polymethylbenzenes to aromatic polycarboxylic acids in the presence of a reaction medium comprising a monocarboxylic acid, such as acetic acid, and a catalyst system comprising one or more heavy metals and bromine.
2. Description of the Prior Art
In U.S. Pat. No. 2,833,816, Saffer, et al, disclose a process for producing an aromatic polycarboxylic acid wherein an aromatic compound selected from polyalkyl aromatic compounds and intermediate oxygenated derivatives thereof is oxidized in a liquid phase with molecular oxygen in the presence of a catalyst comprising bromine and a heavy-metal oxidation catalyst. A reaction medium, such as an aliphatic monocarboxylic acid containing about 1 to 8 carbon atoms, is added when needed.
Kamiya reports in "Tetrahedron"; Vol 22, page 2029 (1966), that the induction period in the cobalt (II) dibromide catalyzed oxidation of tetralin could be eliminated by the addition of sodium acetate.
Hay, et al, report in "Autooxidation Reactions Catalyzed by Cobalt Acetate Bromide", page 1306 (1964) that the initial oxidation of cumene was faster with a cobalt (II) dibromide catalyst than with a cobalt (II) acetate/hydrogen bromide mixture.
Sheherbina and Lysukho report in KINETIKA I KATALIZ, 19, page 1076 (1978) that the addition of metal acetates to the oxidation of durene, pseudocumene, and p-xylene in glacial acetic acid by oxygen at a temperature of 95.degree. C. (203.degree. F.) and atmospheric pressure in the presence of a cobalt bromide catalyst almost eliminated completely relatively long induction periods. Acetates of sodium, potassium, barium, zinc, cobalt, or manganese were found to promote this type of oxidation process in the presence of a cobalt bromide catalyst.
Ariko, Samtsevick, and Mitskevich report in a paper to the Institute of Organophysical Chemistry of the Academy of Sciences, Belorussian SSR that the introduction of magnesium acetate to a cobalt bromide catalyst decreased the rate of carbon dioxide formation during the oxidation of p-xylene in the presence of acetic acid.
As shown hereinafter, the oxidation of pseudocumene in the presence of a catalyst consisting of cobalt tetrafluoroborate and manganese dibromide would initiate briefly and then die. Now it has been found that by using a catalyst comprising at least one non-acetate-containing, heavy-metal compound, a source of bromine, and a non-metal acetate compound as the oxidation catalyst, one can control more easily the rate of oxidation and the selectivity of oxidation by means of the acetate concentration that is used.